Optical filter module and manufacturing method thereof

ABSTRACT

Precise V-grooves allowing an optical fiber to project are formed in respective main surfaces of an upper plane substrate and a lower plane substrate. The optical fiber is mounted on one plane substrate, and a multi-layer film filter  2  is inserted into a filter insertion groove  1  crossing the precise V-groove in the main surface of the other plane substrate. Then, using the projecting optical fiber and the other precise V-groove as a guide rail, two plane substrates grapple the multi-layer film filter to fix and mount the filter. In an optical filter module obtained in this manner, a process of adjusting an optical axis between input/output optical fibers is eliminated, and displacement of the inserted optical filter is reduced.

TECHNICAL FIELD

[0001] The present invention relates to an optical filter module foroptical communication including an optical fiber and an optical waveguide having an optical filter integrated therein, and to a method ofmanufacturing the module.

BACKGROUND ART

[0002] An optical fiber attenuates light due to radiation or absorptionin the fiber in an optical information communication system. An opticalfiber amplifier is employed as a system for amplifying the attenuatedlight. A configuration of the system is shown in FIG. 9A. The opticalfiber amplifier includes an optical isolator 53, an optical fiber 51doped with erbium, a pumping laser 54 for radiating laser beam forexcitation, a photo diode (PD) 52 for monitoring an output, and a filter55 for adjusting a gain. The optical fiber amplifier can amplify a lightoutput without converting an optical signal to an electrical signal,thus being indispensable to an optical communication network.

[0003] A wavelength of the pumping laser 54 in a single mode fiber is1.48 μm. However, the gain depends on a frequency as characteristic A ofFIG. 9B. Therefore, a gain flattening filter (GFF) 55 having acharacteristic reverse to characteristic A, namely having characteristicB, must be inserted into a transmission path after amplifying an opticaloutput to flatten the gain as total characteristic C.

[0004] The GFF 55 is usually inserted between fibers integrally, thusbeing formed in a module. The structure of the module is shown in FIG.9C. Optical fibers 56, condensing lenses 57, and a GFF 58 are mountedand sealed into a metal case 59 after their optical axes are aligned.

[0005] When the module including the GFF 58 integrally mounted betweenthe optical fibers 56 is manufactured, the optical fibers 56 aretemporarily fixed to the metal case 59. The condensing lenses 57 forcondensing light diffused from the optical fibers 56 and the GFF 58 arethen mounted. Then, while optical signals radiated from the opticalfiber 56 at an input side and output from the optical fiber 56 at anoutput side is measured by a power meter, their optical axes are alignedso as to obtain maximum optical signals. Then, the condensing lenses 57and the GFF 58 are caulked and fixed to the metal case 59. Thus, fouroptical axes, i.e., an axis between the optical fiber 56 on the incomingside and the condensing lens 57, an axis between the condensing lens 57and the gain flattening filter 58, an axis between the gain flatteningfilter and the condensing lens 57, and an axis between the condensinglens and the optical fiber 56 on the outgoing side must be aligned,thereby increasing cost.

[0006] Japanese Patent No. 3175814 discloses a system for addressing theproblem discussed above. A structure of the system is shown in FIG. 10Aand FIG. 10B. A multi-layer film filter is inserted in an insertiongroove crossing a built-in optical wave guide formed in a siliconsubstrate, thus providing a multiplexer/demultiplexer for wavelengthdivision multiplexing (WDM). The filter system including a siliconsubstrate 31, a clad member 32, a wave guide 33 for input, a common port331, a first wave guide 34 for output, a first output port 341, a secondwave guide 35 for output, a second output port 351, an insertion groove36 for a multi-layer film filter, and the multi-layer film filter 37.The filter system includes passages guiding light as the wave-guides andis formed in one plane. The filter insertion groove 36 crossing the waveguides allows respective optical axes of the wave guides 33, 35, and 34not to be required to adjust.

[0007] The filter insertion groove 36 having a width (W), a thickness(Df), and a width (Dg) satisfies, with the multi-layer film filter 37having a camber of a curvature radius (R), the relation:

R<W ²/8(Dg−Df).

[0008] Under the relation, the multi-layer film filter 37 can besecurely fixed in the filter insertion groove 36 due to the camber ofthe filter. In other words, the built-in optical wave-guide forms anoptical wave guide path, and a various filter is integrally insertedinto the path.

[0009] However, the multi-layer film filter 37 in this filter systemnecessarily has the camber of a curvature. The camber makes incidenceangle to the multi-layer film filter shift largely due to its mountingdisplacement. The built-in optical wave guide, upon being employed asthe optical wave guide path, creates gaps formed inevitably between thewave guides 33, 35 and the multi-layer film filter 37 in the filterinsertion groove 36. Light diffuses from wave-guides 33, 35 through thegaps and disappears. In addition, it is difficult to mechanically hold acomponent having a camber and to insert it into a micro groove.

SUMMARY OF THE INVENTION

[0010] An optical filter module includes a filter, first and secondoptical fibers having respective cross sections face each other onrespective sides of the filter, a first substrate having a surfacehaving a first groove and a filter insertion groove formed therein, asecond substrate grappling the first optical fiber with the firstsubstrate, and a third substrate grappling the second optical fiber withthe first substrate, grappling the filter with the second substrate, andhaving a third groove formed therein for holding the second opticalfiber therein. The first groove holds the first and second opticalfibers therein, and the filter insertion groove fixes the filter thereinand crossing the first groove. The second substrate has a second grooveformed therein for holding the first optical fiber therein.

[0011] In the optical filter module, the multi-layer film filter can besecurely fixed and mounted to an end surface of the optical fiberwithout relying on a camber of the filter. An upper plane substrategrapples the multi-layer film filter, so that the camber of themulti-layer film filter is corrected, and incident angles for theoptical fiber and the multi-layer film filter are stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1A is a top view of an optical filter module in accordancewith exemplary embodiment 1 of the present invention.

[0013]FIG. 1B is a side view of the optical filter module in accordancewith embodiment 1.

[0014]FIG. 1C is a sectional view of the optical filter module inaccordance with embodiment 1.

[0015]FIG. 2A through FIG. 2D are side views of the optical filtermodule in accordance with embodiment 1.

[0016]FIG. 3A through FIG. 3G show a method of manufacturing an opticalfilter module in accordance with exemplary embodiment 2 of theinvention.

[0017]FIG. 4A through FIG. 4G show another method of manufacturing anoptical filter module in accordance with embodiment 2.

[0018]FIG. 5A through FIG. 5I show a method of manufacturing an opticalfilter module in accordance with exemplary embodiment 3 of theinvention.

[0019]FIG. 6A through FIG. 6I show another method of manufacturing anoptical filter module in accordance with embodiment 3.

[0020]FIG. 7A through FIG. 7I show further method of manufacturing anoptical filter module in accordance with embodiment 3.

[0021]FIG. 8A through FIG. 8I show still another method of manufacturingan optical filter module in accordance with embodiment 3.

[0022]FIG. 9A is a block diagram of a conventional optical amplifyingsystem.

[0023]FIG. 9B shows characteristics of the conventional opticalamplifying system.

[0024]FIG. 9C is a block diagram of the conventional optical filtermodule.

[0025]FIG. 10A and FIG. 10B show a method of mounting the conventionaloptical filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] (Exemplary Embodiment 1)

[0027]FIG. 1A is a twin-core optical filter module in accordance withexemplary embodiment 1 of the present invention. FIG. 1B is a side viewof the optical filter module, and FIG. 1C is a sectional view of theoptical filter module. The module includes an insertion groove 1 for amulti-layer film filter, a multi-layer film filter 2, an upper planesubstrate 3, an optical path 4, a lower plane substrate 5, and anoptical adhesive 6. Respective main surfaces of the upper planesubstrate 3 and the lower plane substrate 5 have V-shaped- grooves 10and 11 formed therein.

[0028] The multi-layer film filter 2 includes, for example, manydielectric films made of SiO₂ or Ta₂O₅ and laminated on a resinsubstrate made of glass or polyimide. The upper plane substrate 3 andthe lower plane substrate 5 are made of glass or silicon, and theirlinear expansion coefficients are preferably close to that of theoptical path 4. A resin-based adhesive, upon fixing the substrates,however, has elasticity to moderate stress caused by difference betweenthe linear expansion coefficients. A photo-curing adhesive, upon beingused, is preferably made of light-transmittable material. The opticaladhesive 6 has a refractive index equivalent to material of the opticalpath 4 so as to prevent light from dispersing from end surfaces of theoptical path.

[0029] If the optical path 4 is made of quartz glass series, refractiveindex of the optical adhesive 6 is preferably close to refractive indexof 1.44 of the quartz glass. The optical path 4 is implemented, forexapmle, by an optical fiber, and the upper plane substrate 3 and thelower plane substrate 5 have V-grooves 10 and 11 for mounting and fixingthe optical fiber, repectively. The V-grooves 10 and 11 have uniformdepths and angles. The V-grooves 10 and 11 having certain depths andangles in the plane substrates 3 and 5 enables the plane substrates 3and 5 to hold the optical fiber without displacement.

[0030] Tip angle α and depth d of the V-grooves 10 and 11 and radius rof the mounted optical fiber satisfies the relation:

d=r/sin(α/2).

[0031] The relation allows the center of the optical fiber to align atthe surfaces of the plane substrates 3 and 5. A clad as the optical pathof a glass-based optical fiber typically has a diameter of 125 μm. Ifeach of the V-grooves 10 and 11 has a tip angle of 90 degrees a depth ofabout 180 μm, the upper and lower plane substrates 3, 5 grapple theoptical fiber, thereby securely fixing the optical fiber.

[0032] The V-grooves 10 and 11 formed in the upper and lower planesubstrates 3, 5 do not necessarily have the same depth and angle. Theangle and depth of the V-groove 11 formed in one plane substrate forgrappling the optical fiber may be more than a projection amount of theoptical fiber from the main surface of another plane substrate havingthe optical fiber mounted in the V-groove 10 formed in anothersubstrate. This arrangement enables the optical fiber to be securelymounted and fixed onto the upper and lower plane substrates 3, 5.However, if the V-grooves 10, 11 formed in the main surfaces of theplane substrates 3, 5 for grappling the optical fiber are larger thannecessary, the upper plane substrate 3 does not function as a guideeffectively in a manufacturing method described below. Therefore, theangles and depths of the V-grooves 10 and 11 ranges appropriately.

[0033] If the V-grooves 10 and 11 have the same tip angles, and if theV-groove 10 formed in one plane substrate has the depth for embeddingthe optical fiber at its center, the depth of the V-groove 11 formed inanother plane substrate should be 10% longer than that of the V-groove10. The filter insertion groove 1 formed in the lower plane substrate 5is aligned by a predetermined angle θ with respect to the optical fiber.The angle θ prevents light incoming through the optical fiber to themulti-layer film filter 2 from being reflected, and depends on thewavelength of guided light. When a single mode fiber (SMF) guides lighthaving a wavelength of 1.48 μm, the angle θ is preferably about 5 to 10degrees.

[0034] A core radius expansion fiber, which includes a core partiallyexpanded for confining and guiding the light, upon used as the opticalfiber, can further reduce the coupling loss between the optical fiberand the multi-layer film filter 2.

[0035] In the optical filter module of embodiment 1, the multi-layerfilm filter 2 is inserted into the filter insertion groove 1 crossingoptical path 4. Therefore, path of the optical fiber is divided by thefilter insertion groove 1, and a gap having a width G identical to thatof the filter insertion groove 1 is provided between end surfaces of theoptical fibers via the multi-layer film filter 2. When a light spotsizes w on the end surfaces of the optical fibers are identical to eachother, a power transmission coefficient Tg through the gap (width G)between the end surfaces of the optical fibers is expressed by theequation:

Tg=[1+{λ×G/(2×π×n×w ²)}²]⁻¹,

[0036] where, n is refractive index of the multi-layer film filter 2,and λ is a wavelength of the light.

[0037] This equation shows that the transmission loss increases inproportion to increase of the wavelength or the gap width G anddecreases rapidly in proportion to the inverse of the square of the spotsize. Thus, the transmission loss decreases according to increasing ofthe spot size at the end surfaces of the optical fibers exposed at wallsof the filter insertion groove 1. The core radius expansion fiber isrepresented by a thermally expanded core (TEC) fiber. The TEC fiber is aspecial fiber having a radius of a core substantially expanded bydiffusing dope elements, such as GeO₂, for controlling a refractiveindex of the core. The dope elements are diffused by heating a portionof a clad of the fiber. The TEC fiber can increase the spot size of thelight from the end surfaces of the optical fibers. In the optical filtermodule of embodiment 1 including the core radius expansion fiber, thefilter insertion groove 1 preferably be machined and formed so as tocross a portion of the largest radius of the expanded core of theoptical fiber.

[0038] The filter insertion groove 1 can have various shapes shown inFIG. 2B through FIG. 2D. In each shape, one wall of the wall is shallow,and the other wall of the groove is deep at the bottom of the filterinsertion groove 1. In FIG. 2B, one wall of the groove is formed fromthe bottom of the filter insertion groove 1 to the main surface of thelower plane substrate 5 at a constant angle θ2. In FIG. 2C, the bottomof the filter insertion groove 1 includes a flat portion. In FIG. 2D,the bottom of the filter insertion groove 1 includes a flat portion, anda part of one wall of the groove slopes at a small angle θ4. The filterinsertion groove 1 is formed with, for example, a diamond grindstonehaving its tip previously formed in the shape of the groove. The shapeof the filter insertion groove 1 is the same as that of the tip of thediamond grindstone, so that the shape of the groove 1 largely depends onthe accuracy of the diamond grindstone. If the width of the grindstoneis 0.1 mm or less, it is difficult to form the groove 1 due to a size ofavailable diamond grain. If the width of the grindstone exceeds 0.1 mm,the groove 1 is relatively easily formed.

[0039] Since the filter insertion groove 1 has one wall sloping andanother wall gentler than one wall, as shown in FIG. 2B to FIG. 2D, themulti-layer film filter 2 after insertion is prevented from tilting.While the filter insertion groove 1 is formed, the end surface of theupper plane substrate 3 having the fiber mounted thereon, i.e., areference surface in a manufacturing method discussed below, receives aforce in a direction that the diamond grindstone is pressed to the upperplane substrate 3 as the reference surface. This operation prevents theforming accuracy of the filter insertion groove 1 from degrading due tomeandering of the diamond grindstone. In above discussion, the moduleincludes two optical fibers, however, the module may include a multiplearray having more fibers.

[0040] (Exemplary Embodiment 2)

[0041] Referring to FIG. 3 and FIG. 4, methods of manufacturing anoptical filter module in accordance with exemplary embodiment 2 of thepresent invention will be described.

[0042] In FIG. 3, the optical filter module includes an insertion groove1 for a multi-layer film filter, a multi-layer film filter 2, an upperplane substrate 3, an optical fiber 4 as an optical path, a lower planesubstrate 5, and an optical adhesive 6, a precise V-groove 10 formed inthe main surface of the lower plane substrate 5, and a precise V-groove11 formed in the main surface of the upper plane substrate 3. Referencenumeral 12 denotes a light source, such as ultraviolet ray, or a heatsource.

[0043] In a method of manufacturing the optical filter module inaccordance with embodiment 2, as shown in FIG. 3A, the upper planesubstrate 3 and the lower plane substrate 5 having precise V-grooves 10and 11 in their main surfaces are prepared. The plane substrates 3 and 5preferably are preferably made of material having a linear expansioncoefficient equivalent to that of the optical fiber 4 mounted and fixedin a subsequent process, and being light-transmittable since the planesubstrates 3 and 5 are fixed with a photo-curing adhesive.

[0044] Then, as shown in FIG. 3B, the optical fiber 4 is mounted andfixed on the lower plane substrate. The mounted portion of the opticalfiber 4 is preferably a clad having no coating in consideration ofmounting accuracy. The angle and depth of the V-groove 10 are determinedso as to project the mounted optical fiber 4 from the main surface ofthe lower plane substrate 5. The optical fiber 4 projecting as a guiderail enables the upper plane substrate 3 to be positioned easily. Thecore radius expansion fiber may be used as the projecting optical fibers4, thereby expanding a spot size at the end surfaces of the opticalfibers 4 and reducing transmission loss or coupling loss between theoptical fibers 4.

[0045] Next, as shown in FIG. 3C, the filter insertion groove 1 crossingthe optical fibers 4 is formed in the main surface of the lower planesubstrate 5 having the optical fiber 4 mounted thereon. The multi-layerfilm filter 2 is inserted into the filter insertion groove 1 in asubsequent process. The optical fibers 4 and the filter insertion groove1 are crossed each other at a certain angle for preventing reflection oflight at a surface of the multi-layer film filter 2. The angle of thecrossing, which depends on the wavelength of light, generally rangesfrom 5 through 10 degrees. If the core radius expansion fiber is used asthe projecting optical fiber 4, the filter insertion groove 1 preferablecrosses a portion having the largest radius of the core.

[0046] The filter insertion groove 1 may be formed by grinding using agrindstone, or by powder machining, such as blasting. The grindstone,for example, is formed in the shape of the groove and employs cBN ordiamond as abrasive grain. The plane substrates may be made of silicon,thus allowing the groove to be formed by wet etching or dry etching. Thecross section of the filter insertion groove 1 formed as discussed inembodiment 1 has its shape relatively freely formed by the grindstone.

[0047] The filter insertion groove 1 is formed simultaneously todividing of the optical fiber 4. For reducing roughness of the endsurfaces of the cut and divided optical fiber 4, an abrasive grain sizeof the grindstone is as small as possible for machining. The filterinsertion groove 1 is formed with a diamond grindstone according toembodiment 2, and the diamond grain of #3000 or more providessubstantially-specular end surfaces of the optical fiber 4.

[0048] Next, as shown in FIG. 3D, the entire main surface of the lowerplane substrate 5 is coated with the optical adhesive 6, and themulti-layer film filter 2 is inserted into the filter insertion groove 1filled with the optical adhesive 6. The multi-layer film filter 2 hasits size for having its upper portion project from the main surface ofthe lower plane substrate 5. The amount of the projected portion of themulti-layer film filter 2 is preferably at least one half or more of thethickness of the upper plane substrate. The optical adhesive 6 is putalso into the filter insertion groove 1 formed in the previous process.The optical adhesive 6 has a refractive index equivalent to that ofmaterial of the core of the optical fiber 4, thereby reducing couplingloss between the fibers. The optical adhesive 6 may be cured with light,such as ultraviolet ray, or heating, however, be preferably cured withthe light for reducing a manufacturing time.

[0049] Then, as shown in FIG. 3E and FIG. 3F, the upper plane substrates3 having the V-grooves 11 in its main surfaces are moved from respectivesides of the multi-layer film filter 2 projecting from the filterinsertion groove 1. At this moment, the optical fiber 4 projecting fromthe main surface of the lower plane substrate 5 functions as a guiderail. The multi-layer film filter 2 inserted into the filter insertiongroove 1 is then grappled from both sides and fixed. In this process,one upper plane substrate 3 has a side surface contact a wall of thefilter insertion groove 1, thereby mounting and fixing the insertedmulti-layer film filter 2 along the wall of the filter insertion groove1. This operation prevents the multi-layer film filter 2 from breakingwhen the filter is grappled with the other upper plane substrate 3.

[0050] The projecting portion of the multi-layer film filter 2 isgrappled. If the multi-layer film filter 2 cambers, the amount of theprojecting portion thereof may be extended to have an area of theportion grappled with the upper plane substrate 3 enlarged to correctthe camber. The multi-layer film filter 2 includes a substrate of glasscan be prevented from breakage by stress since the glass has a largeelastic coefficient. The angle and depth of the V-groove 11 formed inthe main surface of upper plane substrate 3 are determined according tothe amount of the projecting portion of the optical fiber 4. Tolerancesof the angle and depth of the V-groove 11 against the amount of theprojecting portion of the fiber reduces an effect as the guide rail ofthe optical fiber 4. The V-groove 11 smaller than the projecting amountof the optical fiber 4 reduces the effect as the guide rail andreliability. The reliability is reduced since the upper plane substrates3 are mounted to the lower plane substrate 5 inclinedly about theoptical fiber 4.

[0051] Finally, as shown in FIG. 3G, light, such as ultraviolet ray, isradiated or heat is applied from one side or both sides of the upper andlower plane substrates 3, 5 to cure and finish the optical filtermodule. The optical fiber 4 is mounted and fixed to the lower planesubstrate 5, and is then cut simultaneously to forming of the filterinsertion groove 1 in the processes discussed above. This process allowsthe optical axis of the end surfaces of the optical fiber 4 divided intotwo not to be required to align. Additionally, the cutting of theoptical fiber 4 and the forming of the filter insertion groove 1 aresimultaneously performed, thereby reducing manufacturing time.

[0052] In a manufacturing method shown in FIG. 4, a precise V-groove 10is formed in the main surface of the lower plane substrate 5, as shownin FIG. 4A. A filter insertion groove 1 crossing the precise V-groove 10is then formed in the main surface of the lower plane substrate 5, asshown in FIG. 4B. Then, as shown in FIG. 4C, the main surface of thelower plane substrate 5 is coated with an optical adhesive 6, and themulti-layer film filter 2 is inserted into the filter insertion groove1.

[0053] Then, as shown in FIG. 4D, previously divided optical fibers 4are mounted and fixed along the V-groove 10 formed in main surface ofthe lower plane substrate 5. At this moment, the multi-layer film filter2 contacts a wall of the filter insertion groove 1, thus being preventedfrom breakage when the multi-layer film filter 2 is grappled with upperplane substrates 3. In this case, the optical fibers 4 have the endsurfaces finished previously specularly, thus having a reducedtransmission loss and coupling loss between the optical fibers 4. Theend surfaces of the optical fibers 4 contact the main surface of theinserted multi-layer film filter 2, thereby further reducing thetransmission loss and coupling loss between the optical fibers 4.Processes shown in FIG. 4E through FIG. 4G are the same as those of FIG.3E through FIG. 3G, thus not being described.

[0054] (Exemplary Embodiment 3)

[0055] Referring to FIG. 5A through FIG. 8I, methods of manufacturingoptical filter modules in accordance with exemplary embodiment 3 of thepresent invention will be described.

[0056] In a method of manufacturing the optical filter module inaccordance with exemplary embodiment 3, firstly, plane substrates 3 and5 having precise V-grooves 10 and 11 in their main surfaces,respectively, are prepared, as shown in FIG. 5A. The plane substrates 3and 5 is preferably made of material having a linear expansioncoefficient equivalent to that of an optical fiber 4 mounted and fixedin a subsequent process. The material may preferably belight-transmittable since the upper and lower plane substrates 3, 5 arefixed with a photo-curing adhesive.

[0057] Then, as shown in FIG. 5B, the optical fiber 4 is mounted andfixed in the precise V-groove 11 formed in the main surface of the upperplane substrate 3. The mounted portion of the optical fiber 4 maypreferably includes a clad having no coating, such as resin, inconsideration of the mounting accuracy. The angle and depth of theV-groove 11 are determined to allow the mounted and fixed optical fiber4 to project from the main surface of the lower plane substrate 5. Theupper plane substrate 3 is thus easily positioned using the projectingoptical fiber 4 as a guide rail. A core radius expansion fiber may beused as the projecting optical fiber 4, thereby expanding a spot size atthe end surfaces of the optical fiber 4 and reducing the transmissionloss or coupling loss between the optical fibers 4.

[0058] Next, as shown in FIG. 5C, the upper plane substrate 3 havingoptical fiber 4 mounted and fixed thereon in the previous process is cutand divided. If the core radius expansion fiber is used as the opticalfiber 4, a portion having the largest radius of a core is cut. Finishingthe cut surface of the optical fibers 4 in specular reduces the couplingloss between the optical fibers 4. When the cut surfaces are used asthey are, the cutting condition must be considered. If the upper planesubstrate 3 is cut with a diamond grindstone or the like, the grindstonehas small abrasive grain size determined so that the end surface of cutoptical fibers 4 is as specular as possible. According to thisembodiment, the diamond grindstone is used to form a filter insertiongroove 1. The diamond grindstone includes diamond grain of #3000 ormore, thereby allowing the end surfaces of the optical fibers 4 to besubstantially specular.

[0059] Next, as shown in FIG. 5D, the filter insertion groove 1 crossingthe precise V-groove 10 at a certain angle is formed in the main surfaceof the lower plane substrate 5. The crossing angle, which is determinedaccording to the wavelength of employed light, generally ranging from 5to 10 degrees. The filter insertion groove 1 may be formed by grindingwith a grindstone, or by powder machining, such as blasting. Thegrindstone, for example, has the shape of the groove and employs cBN anddiamond as abrasive grain. The plane substrates made of silicon enablingthe groove 1 to be formed by wet etching or dry etching. When the crosssection of the filter insertion groove 1 is machined as discussed inembodiment 1, the shape is formed relatively freely with the grindstone.

[0060] Next, as shown in FIG. 5E, the entire main surface of the lowerplane substrate 5 is coated with an optical adhesive 6. At this moment,the optical adhesive 6 is put also into the filter insertion groove 1formed in the previous process. The optical adhesive 6 has a refractiveindex equivalent to that of material of a core of the optical fibers 4,thereby reducing the coupling loss between the optical fibers 4. Theoptical adhesive 6 is cured with light, such as ultraviolet ray, orheating. The curing with the light is more advantageous for reducingmanufacturing time.

[0061] Then, as shown in FIG. 5F, the multi-layer film filter 2 isinserted into the filter insertion groove 1 filled with the opticaladhesive 6. The multi-layer film filter 2 has its size to allowing theupper part of the inserted multi-layer film filter 2 to project from themain surface of the lower plane substrate 5. The amount a projectingportion of the multi-layer film filter 2 is preferably at least one halfor more of the thickness of the upper plane substrate 3.

[0062] Then, as shown in FIG. 5G and FIG. 5H, the upper plane substrates3 having the V-groove 11 in the main surface are moved from respectivesides of the multi-layer film filter 2 projecting from the filterinsertion groove 1. At this moment, the optical fiber 4 projecting fromthe main surface of the lower plane substrate 5 functions as a guiderail. The multi-layer film filter 2 inserted into the filter insertiongroove 1 is grappled from their both sides and fixed. In this process,one upper plane substrate 3 is mounted so that a side surface of theupper plane substrate 3 is aligned to a wall of the filter insertiongroove 1, thereby mounting and fixing the inserted multi-layer filmfilter 2 on the wall of the filter insertion groove 1. This operationprevents the multi-layer film filter 2 from breakage when the filter isgrappled with the other upper plane substrate 3.

[0063] The multi-layer film filter 2 is grappled at its projectingpotion. If the multi-layer film filter 2 cambers, the amount of itsprojecting portion has an enlarged area of a portion grappled by theupper plane substrate 3 to correct the camber. The multi-layer filmfilter 2 including a substrate of glass can be prevented from breakagedue to stress since the glass has a large elastic coefficient. The angleand depth of the V-groove 11 formed in the main surface of each upperplane substrate 3 are determined according to the amount of theprojecting portion of the optical fiber 4. Tolerances of the angle anddepth of the V-groove 10 for the projecting amount reduces an effect asthe guide rail of the optical fiber 4. The V-groove 10 smaller than theprojecting amount of the optical fiber 4 reduces the effect as the guiderail and reliability. The reliability may is reduced since the upperplane substrates 3 are mounted to the lower plane substrate 5 inclinedlyabout the optical fiber 4.

[0064] Finally, as shown in FIG. 5I, light 12, such as ultraviolet ray,is radiated, or heat 12 is applied from one side or both sides of theupper and lower plane substrates 3, 5 to cure and finish the opticalfilter module. In the manufacturing method of the present embodiment,the upper plane substrates 3 previously having the optical fibers 4mounted thereon are prepared as discussed above. This preparationenables the process of applying the optical adhesive 6, the process ofinserting the multi-layer film filter 2, and the process of grapplingthe multi-layer film filter 2 by the upper plane substrates 3 to beexecuted continuously, and facilitates manufacturing control.

[0065] In a manufacturing method shown in FIG. 6A through FIG. 6I, aprecise V-groove 11 is formed in the main surface of an upper planesubstrate 3, as shown in FIG. 6A. The upper plane substrate 3 is thendivided into two, as shown in FIG. 6B, and two optical fibers 4 aremounted to the respective upper plane substrates 3 divided so that anend surface of each optical fiber 4 fits to an end surface of each upperplane substrate 3. A special processing, such as specular finish orspherical polishing, can be applied to the end surfaces of the opticalfibers 4, so that the coupling loss between the optical fibers 4 canadvantageously reduced. The subsequent processes shown in FIG. 6Cthrough FIG. 6I are the same as those of FIG. 5C through FIG. 5I, andtheir descriptions are therefore omitted.

[0066] In a manufacturing method shown in FIG. 7A through FIG. 7I, afilter insertion groove 1 crossing a precise V-groove 10 at a certainangle is formed in the main surface of the lower plane substrate 5.Processes shown in FIG. 7A through FIG. 7D are the same as those of FIG.5A through FIG. 5D. In FIG. 7E, one upper plane substrate 3 having theoptical fiber 4 mounted thereon is guided by the precise V-groove 10formed in the main surface of the lower plane substrate 5 and contactsthe main surface of the lower plane substrate 5. The upper planesubstrate 3 is positioned and fixed so that the end surface of theoptical fiber 4 mounted on the upper plane substrate 3 contacts a wallof the filter insertion groove 1.

[0067] As shown in FIG. 7E, an optical adhesive 6 is then applied to themain surface of the lower plane substrate 5 before having upper planesubstrate 3 mounted thereon. At this moment, the optical adhesive 6 isalso put into the filter insertion groove 1. In FIG. 7F, a multi-layerfilm filter 2 is then inserted into the filter insertion groove 1 tocontact the end surface of the fixed upper plane substrate 3. Themulti-layer film filter 2 contacts the filter insertion groove 1 and theend surface of the upper plane substrate 3, thus being prevented frombreaking when the filter is grappled with the other upper planesubstrate 3.

[0068] As shown in FIG. 7G and FIG. 7H, the other upper plane substrate3 is then disposed on a portion of the main surface of the lower planesubstrate 5 having no upper plane substrate 3 mounted thereon. The upperplane substrate 3 moves so as to make the optical fiber 4 projectingfrom the main surface of the upper plane substrate 3 slide along theprecise V-groove formed in the main surface of the lower plane substrate5. Then, the substrate 3 abuts on the multi-layer film filter 2 insertedinto the filter insertion groove 1 to grapple and fix the filter 2.

[0069] In FIG. 7I, finally, light 12, such as ultraviolet ray, isradiated, or heat 12 is applied from one side or both sides of the upperand lower plane substrates 3, 5 to cure and finish the optical filtermodule. The inserted multi-layer film filter 2 is fixed on the upperplane substrate 3 and the wall of the filter insertion groove 1, thusbeing prevented from breaking. Since one upper plane substrate 3 ispreviously fixed, the other upper plane substrate 3 is easilypositioned.

[0070] Another manufacturing method shown in FIG. 8A through FIG. 81includes processes shown in FIG. 8A through FIG. 8C which are the sameas those in the manufacturing method shown in FIG. 7. In FIG. 8D, upperplane substrate 3 having an optical fiber 4 mounted thereon is mountedand fixed on the main surface of the lower plane substrate 5. The lowerplane substrate 5 is mounted and fixed so that the optical fiber 4projecting from the main surface of the upper plane substrate 3 istaken, as a guide rail, along a precise V-groove 10 formed in the mainsurface of the lower plane substrate 5. In FIG. 8E, a filter insertiongroove 1 is formed in the main surface of the lower plane substrate 5along an end surface of the mounted upper plane substrate 3 at a certainangle with respect to a precise V-groove 11 formed in the main surface.

[0071] In the manufacturing method shown in FIG. 8A through FIG. 8E, thefilter insertion groove 1 is formed after one upper plane substrate 3 ismounted and fixed on the main surface of the lower plane substrate 5,thus allowing the upper plane substrate 3 to be easily positioned. Themethod shown in FIG. 8A through FIG. 8E can be applicable to themanufacturing methods shown in FIG. 3A through FIG. 3G and shown in FIG.4A through FIG. 4G. In other words, the optical fiber 4 is mounted onthe lower plane substrate 5, then one upper plane substrate 3 having theprecise V-groove 11 in its main surface is mounted and fixed. The filterinsertion groove 1 is formed in the lower plane substrate 5 withreference to an end surface of the upper plane substrate 3. Thesubsequent processes shown in FIG. 8F through FIG. 8I are the same asthose in the manufacturing method shown in FIG. 7, and therefore, theirexplanations are omitted.

Industrial Applicability

[0072] According to the present invention, precise V-grooves are formedin main surfaces of upper and lower plane substrates, and an opticalfiber is mounted in the V-grooves, thereby allowing the optical fiber tome mounted easily on the main surface. The precise v-groove formed inthe main surface of one plane substrate functions as a guide rail, theother plane substrate grapples a multi-layer film filter inserted intoan insertion groove. This arrangement provides an optical filter modulein which shape deformation, such as camber, of the multi-layer filmfilter does not affect, the upper plane substrate is easily positioned,and adjustment of an optical axis is not required.

1. An optical filter module comprising: a filter; first and secondoptical fibers having respective cross sections face each other onrespective sides of said filter; a first substrate having a surfacehaving a first groove and a filter insertion groove formed therein, saidfirst groove holding said first and second optical fibers therein, saidfilter insertion groove fixing said filter therein and crossing saidfirst groove; a second substrate grappling said first optical fiber withsaid first substrate, said second substrate having a second grooveformed therein for holding said first optical fiber therein; and a thirdsubstrate grappling said second optical fiber with said first substrate,grappling said filter with said second substrate, and having a thirdgroove formed therein for holding said second optical fiber therein. 2.The optical filter. module according to claim 1, wherein said first andsecond optical fibers comprises core radius expansion fibers.
 3. Theoptical filter module according to claim 1, wherein respective endsurfaces of said first and second optical fibers facing said filter haveexpanded radiuses of cores thereof.
 4. The optical filter moduleaccording to claim 1, wherein said filter abuts on a wall of said filterinsertion groove.
 5. The optical filter module according to claim 1,wherein said filter insertion groove has one wall and another walldeeper than said one wall.
 6. The optical filter module according toclaim 1, wherein said first through three substrates and said first andsecond optical fibers are coupled through one of a photo-curing adhesiveand a heat curing adhesive.
 7. A method of manufacturing an opticalfilter module, comprising the steps of: providing a first substratehaving a first groove formed in a surface thereof; providing second andthird substrates having second and third grooves formed in surfacesthereof, respectively; disposing an optical fiber in the groove to allowa portion of the optical fiber to project from the surface of the firstsubstrate; forming a filter insertion groove crossing the optical fiberafter said step of disposing the optical fiber; inserting a filter intothe filter insertion groove; disposing the second substrate on thesurface of the first substrate through engaging the portion of theoptical fiber projecting from the surface with the second groove;disposing the third substrate on the surface of the first substratethrough engaging the portion of the optical fiber projecting from thesurface with the third groove; grappling the filter inserted into thefilter insertion groove with the second and third substrates; and fixingthe second and third substrates to the first substrate.
 8. The methodaccording to claim 7, wherein said step of grappling the filtercomprises the sub-step of moving the second substrate disposed on thefirst substrate toward the filter along the optical fiber.
 9. The methodaccording to claim 8, wherein said step of grappling the filter furthercomprises the sub-step of moving the third substrate disposed on thefirst substrate toward the filter along the optical fiber.
 10. Themethod according to claim 9, wherein said sub-step of moving the thirdsubstrate is executed after said sub-step of moving the secondsubstrate.
 11. The method according to claim 7, further comprising thestep of: cutting the optical fiber simultaneously to said step offorming the filter insertion groove.
 12. The method according to claim7, wherein said step of cutting the optical fiber comprises the sub-stepof cutting the optical fiber at a portion having a core of the fiberhaving an expanded radius.
 13. A method of manufacturing an opticalfilter module, comprising the steps of: providing a first substratehaving a first groove and a filter insertion groove crossing the firstgroove formed in a surface thereof; providing second and thirdsubstrates having second and third grooves formed in surfaces thereof,respectively; disposing first and second optical fibers in the firstgroove at both sides of the filter insertion groove for allowing theoptical fibers to project from the surface of the first substrate and tohave respective ends of the optical fibers to be aligned at the filterinsertion groove; inserting a filter into the filter insertion groove;disposing the second substrate on the first substrate through engagingthe portion of the first optical fiber projecting from the surface withthe second groove; disposing the third substrate on the first substratethrough engaging the portion of the second optical fiber projecting fromthe surface with the third groove; grappling the filter with the secondand third substrates; and fixing the second and third substrates to thefirst substrate after said step of grappling the filter inserted intothe filter insertion groove.
 14. The method according to claim 13,wherein said step of grappling the filter comprises the sub-step ofmoving the second substrate toward the filter along the first opticalfiber after said step of disposing the second substrate.
 15. The methodaccording to claim 14, wherein said step of grappling the filter furthercomprises the sub-step of moving the third substrate toward the filteralong the second optical fiber after said step of disposing the thirdsubstrate.
 16. The method according to claim 15, wherein said sub-stepof moving the third substrate is executed after said sub-step of movingthe second substrate.
 17. A method of manufacturing an optical filtermodule, comprising the steps of: providing a first substrate having afirst groove and a filter insertion groove crossing the first groove;providing a second substrate having a second groove; disposing anoptical fiber in the second groove; dividing the substrate into thirdand fourth substrates; inserting a filter into the filter insertiongroove; disposing the third and fourth substrates on the firstsubstrate; disposing the optical fiber disposed in the second grooveinto the first groove; grappling the filter inserted into the filterinsertion groove with the third and fourth substrates; and fixing thethird and fourth substrates to the first substrate after said step ofgrappling the filter.
 18. The method according to claim 17, wherein saidstep of grappling the filter comprises the sub-step of moving the thirdsubstrate disposed on the first substrate toward the filter insertedinto the filter insertion groove.
 19. The method according to claim 18,wherein said step of grappling the filter further comprises the sub-stepof moving the fourth substrate disposed on the first substrate towardthe filter.
 20. The method according to claim 19, wherein said sub-stepof moving the fourth substrate is executed after said sub-step of movingthe third substrate.
 21. The method according to claim 17, furthercomprising the step of: cutting the optical fiber simultaneously to saidstep of dividing the second substrate.
 22. A method of manufacturing anoptical filter module, comprising the steps of: providing a firstsubstrate having a first groove and a filter insertion groove crossingthe first groove; providing second and third substrates having secondand third grooves, respectively; inserting a filter into the filterinsertion groove; disposing a first optical fiber in the second grooveto allow an end of the first optical fiber to fit to an end surface ofthe second substrate; disposing a second optical fiber in the thirdgroove to allow an end of the second optical fiber to fit to an endsurface of the third substrate; disposing the second and thirdsubstrates on the first substrate to allow the first and second opticalfibers to be disposed in the first groove; grappling the filter insertedinto the filter insertion groove with the second and third substrates;and fixing the second and third substrates to the first substrate. 23.The method according to claim 22, wherein said step of grappling thefilter comprises the sub-step of moving the second substrate toward thefilter.
 24. The method according to claim 23, wherein said step ofgrappling the filter further comprises the sub-step of moving the thirdsubstrate toward the filter.
 25. The method according to claim 24,wherein said sub-step of moving the third substrate is executed aftersaid sub-step of moving the second substrate.